A main stop valve installed at an inlet of a steam turbine or the like is capable of being closed quickly to block steam and being opened gradually during startup to adjust the supply amount of steam. A trip and throttle (T&T) valve is generally used as such a main stop valve. For example, in the case of a hydraulic-actuation type T&T valve, hydraulic oil is supplied to a cylinder during operation, and the valve is kept open by applying a hydraulic pressure exceeding the pressing force of a spring to a piston. In an emergency, a path configured to rapidly drain the hydraulic oil from the cylinder is opened, and the piston to which the hydraulic pressure is not applied is pushed down by the pressing force of the spring, thereby closing the valve quickly.
In the meantime, when a hydraulic-actuation type T&T valve is started, the valve is gradually opened by a predetermined procedure. For example, in the case of the T&T valve in the related art described in Patent Documents 1 and 2, a valve rod is rotated by a handle operation before the supply of the hydraulic oil in a state where the valve is closed, and the action of a screw installed in the valve rod lifts (resets) the piston to a topmost portion of a cylinder. Subsequently, the hydraulic oil is supplied into the cylinder. At this time point, since the piston is already at the topmost portion of the cylinder, the piston is not raised by the hydraulic pressure, and thus, the valve remains closed. Thereafter, when the valve rod is rotated in the reverse direction by the handle operation, the valve rod is released from the piston held at the topmost portion of the cylinder by the hydraulic pressure by the action of the screw, and the valve is gradually opened.
As a problem in the above-described T&T valve, Patent Documents 1 and 2 point out that, unlike a predetermined procedure, when the hydraulic oil is supplied into the cylinder before the piston is reset, the valve is opened against the operator's will. In Patent Document 1, it has been proposed to provide a bypass flow path and a spring support sleeve engaged with a piston with respect to this problem. Meanwhile, in Patent Document 2, it has been proposed to provide a pilot relay valve configured to selectively supply hydraulic oil to a cylinder, and to move a plunger provided in a main body of the relay valve to a valve rod or displace the plunger manually.